1. Field of the Invention
The present invention relates to a Doherty power amplification apparatus and method using a combined cell.
2. Description of the Related Art
A Doherty power amplifier has recently been proposed for improving the efficiency of a power amplification apparatus. The Doherty power amplifier has a structure in which a carrier power amplifier and at least one peaking power amplifier are combined in parallel. A power amplification apparatus including a single carrier power amplifier and (N−1) peaking power amplifiers is referred to as an “N-way Doherty power amplification apparatus.”
FIG. 1 is a block diagram illustrating a configuration of an N-way Doherty power amplification apparatus according to the related art.
Referring to FIG. 1, an N-way Doherty power amplification apparatus includes a power splitter 100, a carrier power amplifier 102, (N−1) peaking power amplifiers 103 to 104, first to Nth offset transmission lines 122 to 124, a final output end 112, and a power combiner 106. Power combiner 106 includes first and second impedance transformers 108 and 110.
Power splitter 100 splits an input power into N powers, which have a phase difference between them of ‘90’ degrees. It then outputs the N split powers to carrier power amplifier 102 and (N−1) peaking power amplifiers 103 to 104, respectively. Thereafter, carrier power amplifier 102 and (N−1) peaking power amplifiers 103 to 104 amplify the respective input powers, and output the amplified powers to the power combiner 106 through first to Nth offset transmission lines 122 to 124, respectively.
A high input DC bias having a value equal to or larger than a threshold value is used for an input terminal (i.e., gate or emitter) of carrier power amplifier 102. Therefore, carrier power amplifier 102 operates regardless of the magnitude of the input power. On the contrary, a low input DC bias having a value smaller than the threshold value is used for an input terminal of each of (N−1) peaking power amplifiers 103 to 104. Therefore, only when the input power is a high power having a power magnitude equal to or larger than a reference power, each of (N−1) peaking power amplifiers 103 to 104 amplifies the input power.
Namely, when the input power is a low power having a magnitude smaller than the reference power, only carrier power amplifier 102 operates. When the input power is a high power, carrier power amplifier 102 and (N−1) peaking power amplifiers 103 to 104 operate simultaneously. Accordingly, when the input power has a high Peak-to-Average Ratio (PAR), the input power can be amplified with high drain efficiency. The N-way Doherty power amplification apparatus operates in order to have a maximum efficiency in an output that is smaller than maximum output by −20×log(N) decibels (dB).
The Nth offset transmission line 124 controls an output direction of power provided by carrier power amplifier 102, so that the power provided by carrier power amplifier 102 may not flow into (N−1) peaking power amplifiers 103 to 104, but may be provided to power combiner 106. In addition, each of the first to (N−1) offset transmission lines 122 to 123 controls an output direction of power provided by a relevant peaking power amplifier so that power provided by the relevant peaking power amplifier of (N−1) peaking power amplifiers 103 to 104 may not flow into carrier power amplifier 102 and other peaking power amplifiers, but may be provided to power combiner 106.
Power combiner 106 combines power provided by carrier power amplifier 102 with power provided by each of the (N−1) peaking power amplifiers 103 to 104, and provides the combined power to the final output end 112.
First, second impedance transformer 110 has a characteristic impedance value RO/√{square root over (N)} (where N represents an integer obtained by adding the number of carrier power amplifiers and that of peaking power amplifiers). In addition, when a load impedance (hereinafter, referred to as “combined load impedance”) is defined at a point where there is provided combined power which is generated by combining first power provided at the output end of first impedance transformer 108 and second power provided by each of (N−1) peaking power amplifiers 103 to 104, second impedance transformer 110 adjusts a value ZL of the combined load impedance, so that combined load impedance value ZL may become 1/N times (i.e., RO/N) as much as a load impedance (hereinafter, referred to as “output load impedance”) value of final output end 112. First impedance transformer 108 has a characteristic impedance value RO, and changes a load impedance value ZC at an output end of carrier power amplifier 102.
FIGS. 2A and 2B illustrate an operation of an N-way Doherty power amplification apparatus according to the related art.
Referring to FIGS. 2A and 2B, when carrier power amplifier 102 and (N−1) peaking power amplifiers 103 to 104 all operate with the high power input, first impedance transformer 108 sets a value ZC of a load impedance, which is defined at the output end of carrier power amplifier 102, to a value RO corresponding to the output load impedance value.
In addition, when only carrier power amplifier 102 operates with the low power input, first impedance transformer 108 sets value ZC of the load impedance, which is defined at the output end of carrier power amplifier 102, to a value N·RO which is N times as much as the output load impedance value.
When power combiner 106 changes a load impedance at the output end of carrier power amplifier 102 according to whether peaking power amplifier 104 operates, it is referred to as “load modulation”. According to the load modulation, the gain of finally-output power in a case of low power input becomes N times as large as in a case of high power input. Therefore, N-way Doherty power amplification apparatus can have linear input/output.
Therefore, a need exists for a Doherty power amplification apparatus and method in which at least one carrier power amplifier and at least one peaking power amplifier are combined on a cell-by-cell basis respectively, and which can output higher power without power loss.